European Patent Application EP0202375 discloses the use of a passive oscillatory circuit to measure changes in the pressure of a tyre. The natural frequency of the disclosed circuit depends on the capacitive value of the circuit which changes with changes in pressure in the tyre. The natural frequency is measured by inductive coupling to coils outside the tyre. Using resonant circuits to measure the pressure in a tyre can be advantageous because such circuits are cheap to manufacture and generally reliable, in particular since they do not require an internal power source. However, a measuring system such as the one described in EP0202375 is far from ideal. The system is difficult to install or replace, requiring components to be positioned inside the wall of the tyre in order for the pressure to act upon them. The components are also vulnerable to damage while the tyre is in use. It would be desirable to provide a system that was both easier to install and more durable, as well as being more accessible both for use and for maintenance. Other systems for monitoring tyre pressure remotely are also disclosed in EP0108176 and DE4242861